Sputtering target, thin film for optical information recording medium and process for producing the same

ABSTRACT

A sputtering target is provided that has a relative density of 80% or more and contains a compound having as its principal component zinc oxide satisfying A X B Y O (KaX+KbY)/2 (ZnO) m , 1&lt;m, X≦m, 0&lt;Y≦0.9, X+Y=2, where A and B are respectively different positive elements of trivalence or more, and the valencies thereof are respectively Ka and Kb. A ZnO based sputtering target is obtained which does not contain ZnS and SiO 2 , and, upon forming a film via sputtering, is capable of reducing the affect of heating the substrate, of performing high speed deposition, of adjusting the film thickness to be thin, of reducing the generation of particles (dust) and nodules during sputtering, of improving the productivity with small variation in quality, and which has fine crystal grains and a high density of 80% or more, particularly 90% or more.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/178,957 filed on Jul. 24, 2008 and issued as U.S. Pat. No. 7,718,095B2, filed Sep. 2, 2005, which is a divisional of U.S. application Ser.No. 10/547,815 (U.S. Pat. No. 7,635,440 B2) which is the National Stageof International Application No. PCT/JP04/01051, filed Feb. 3, 2004,which claims the benefit under 35 USC §119 of Japanese Application No.2003-056884, filed Mar. 4, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to a sputtering target which allows directcurrent (DC) sputtering upon forming a film via sputtering, has minimalarcing during sputtering, is capable of reducing particles (dust) andnodules resulting therefrom, has high density and small variation inquality, and is capable of improving the productivity, as well as to themanufacturing method thereof. The present invention also relates to athin film for an optical information recording medium (particularly usedas a protective film) obtained by using the said target as well as tothe manufacturing method thereof.

In recent years, technology of high density optical recording disks suchas high density optical information recording mediums capable ofrewriting without requiring a magnetic head has been developed, andthese disks are rapidly attracting attention. This optical disc can beclassified into the three categories of ROM (read-only), R (write-once)and RW (rewritable). Particularly, the phase change method employed inthe RW type discs is attracting attention. The recording principleemploying this phase change optical disk is briefly explained below.

This phase change optical disc performs the recording/reproduction ofinformation by heating and increasing the temperature of a recordingthin film on a substrate by irradiating a laser beam thereto, andgenerating a crystallographic phase change (amorphous

crystal) in the structure of such recording thin film. Morespecifically, the reproduction of information is conducted by detectingthe change in the reflectivity caused by the change in the opticalconstant of the phase.

The aforementioned phase change is performed with the irradiation of alaser beam narrowed down to a diameter of approximately several hundrednm to several μm. Here, for example, when a 1 μm laser beam passesthrough at a linear velocity of 10 m/s, light is irradiated to a certainpoint on the optical disc for 100 ns, and it is necessary to perform theaforementioned phase change and detect the reflectivity within such timeframe.

Moreover, in order to realize the foregoing crystallographic phasechange; that is, the phase change of the amorphous and crystal, not onlywill the recording layer be subject to repeated heating and rapidcooling, the peripheral dielectric protective layer and the reflectivefilm of aluminum alloy will also be repeatedly subject thereto.

In light of the above, a phase change optical disc has a four-layerstructure wherein, for instance, both sides of a Ge—Sb—Te recording thinfilm layer or the like are sandwiched with a ZnS—SiO₂ high-melting pointdielectric or the like, and an aluminum alloy reflective layer isadditionally provided thereto.

Among the above, in addition to being demanded of an optical functioncapable of increasing the absorption of a laser beam at the amorphousportion and crystal portion of the recording layer, and which has alarge reflectivity difference, the reflective layer and protective layerare also demanded of a function for preventing the deformation caused bythe moisture resistance or heat of the recording thin film as well as afunction for controlling the thermal conditions upon recording (c.f.“Kogaku” magazine, volume 26, no. 1, pages 9 to 15).

As described above, the protective layer of a high-melting pointdielectric must be durable against repeated thermal stress caused by theheating and cooling, must not allow such thermal effect to influence thereflective film or other areas, and it is also required to be thin, oflow reflectivity, and possess strength to prevent deterioration. Fromthis perspective, the dielectric protective layer plays an importantrole.

The dielectric protective layer described above is usually formed withthe sputtering method. This sputtering method makes a positive electrodesubstrate and a negative electrode target face each other, and generatesan electric field by applying a high voltage between the substratesthereof and the targets under an inert gas atmosphere. The sputteringmethod employs a fundamental principle where the inert gas are ionized,plasma which consists of electrons and the positive ion is formed, thepositive ion in this plasma extrudes the atoms structuring the target bycolliding with the target (negative electrode) surface, and the extrudedatoms adhere to the opposing substrate surface, wherein the film isformed thereby.

Conventionally, ZnS—SiO₂ has been widely used as the protective layer ofa rewritable optical recording medium due to its superiorcharacteristics regarding optical characteristics, heat characteristics,and adhesiveness with the recording layer. Nevertheless, a rewritableDVD is demanded of increased number of rewritings in addition to therealization of shorter wavelengths of the laser wavelength, realizationof a large capacity and high speed recording are also strongly demanded,and the characteristics of conventional ZnS—SiO₂ are becominginsufficient.

As one reason that the number of times the optical information recordingmedium can be rewritten will deteriorate, there is a problem in that thesulfur constituent from the ZnS—SiO₂ will be diffused to the recordinglayer material disposed between ZnS—SiO₂. Also, pure Ag or Ag alloyhaving high reflectivity and high thermal conductance characteristicsfor realizing large capacity and high speed recording is being used asthe reflective layer material.

This reflective layer is also disposed adjacent to the ZnS—SiO₂protective layer material, and, due to the diffusion of the sulfurconstituent from ZnS—SiO₂, the pure Ag or Ag alloy reflective layermaterial would become corroded and deteriorate, and caused thedeterioration in the characteristics of the reflectivity and so on ofthe optical information recording medium.

Although an intermediate layer having nitride or carbide as itsprincipal component is provided between the reflective layer andprotective layer, and between the recording layer and protective layerin order to prevent the diffusion of the sulfur constituent, there areproblems in that the throughput will deteriorate and costs will increaseas a result of the increased number of layers.

In order to overcome these problems, a material having similarcharacteristics as with the ZnS—SiO₂ protective layer material that doesnot contain ZnS is being sought. Further, since SiO₂ often causes thedeterioration in the deposition rate and abnormal electrical discharge,it is desirable to avoid adding the same.

In light of the above, the use of a material having as its principalcomponent a ZnO base homologous compound (c.f. technical journal “SolidPhysics” C. Li et al., Vol. 35, No. 1, 2000, page 23 to 32 “MicroscopicObservation of Modulated Structure of Homologous Compound RMO₃(ZnO)_(m)(R═In, Fe; M═In, Fe, Ga, Al; m=natural number)”) that does not containZnS and SiO₂ has been considered.

Since a ZnO base homologous compound has a complex layer structure, itis characterized in that it is capable of stably retaining the amorphousnature during deposition, and in this respect, this compound yields thesame effect as the addition of SiO₂. Further, this is transparent in theused wavelength range, and the refractive index is also similar toZnS—SiO₂.

As described above, by reducing or eliminating the influence of thesulfur constituent by using a material having oxide as its principalcomponent as a substitute for a ZnS—SiO₂ protective layer material, thishas been expected to improve the characteristics of the opticalinformation recording medium, and improve the productivity thereof.

Generally speaking, as examples of using a material having a homologouscompound as its principal component as a transparent conductivematerial, for instance, there is a method of forming a zinc-indium oxidetarget via laser abrasion (c.f. Japanese Patent Laid-Open PublicationNo. 2000-26119), an example of a transparent conductive film containingamorphous nature oxide and having favorable conductivity and inparticular favorable blue light permeability (c.f. Japanese PatentLaid-Open Publication 2000-44236), and an example of a moistureresistance film forming target having In and Zn as its principalcomponents, which is In₂O₃(ZnO₂)_(m) (m=2 to 20), and the atomic ratioof In and Zn(In/(In+Zn)) is 0.2 to 0.85 (c.f. Japanese Patent No.2695605).

Nevertheless, it could not be said that the material forming theforegoing transparent conductive film was sufficient as a thin film foran optical information recording medium (in particular for use as aprotective film). Meanwhile, with the ZnO based homologous compound,there is a problem in that it is difficult to increase the bulk density,and only a low density sintered body target can be obtained.

With this kind of low density target, there are problems in that arcingeasily occurs during the formation of the film via sputtering, particles(dust) and nodules will generate during sputtering as a result of sucharcing, and, not only will the uniformity and quality of depositiondeteriorate, the productivity will also deteriorate.

SUMMARY OF THE INVENTION

The present invention relates to a ZnO based sputtering target that doesnot contain ZnS and SiO₂, and, an object of the present invention is toprovide a sputtering target which upon forming a film via sputtering, iscapable of reducing the affect of heating the substrate, performing highspeed deposition, adjusting the film thickness to be thin, reducing thegeneration of particles (dust) and nodules during sputtering, improvingthe productivity with small variation in quality, and which has finecrystal grains and a high density of 80% or more, particularly 90% ormore. In particular, the object of the present invention is to obtain athin film for an optical information recording medium optimal for use asa protective film, as well as the manufacturing method thereof.

In order to achieve the foregoing objects, as a result of intense study,the present inventors discovered that by adjusting the component of acompound having zinc oxide as its principal component and increasing thedensity thereof, characteristics as the protective film can bemaintained, the generation of particles and nodules upon sputtering canbe reduced, and the uniformity of the film thickness can also beimproved.

Based on the foregoing discovery, the present invention provides asputtering target having a relative density of 90% or more andcontaining a compound having as its principal component zinc oxidesatisfying A_(X)B_(Y)O_((KaX+KbY)/2)(ZnO)_(m), where 1<m, X≦m, 0<Y≦0.9,X+Y=2, and where A and B are respectively different positive elements oftrivalence or more, and the valencies thereof are respectively Ka andKb. As shown by Examples 1-5 in Table 1, m can be greater than or equalto 4, and as shown by Examples 1 and 3-5 in Table 1, m can be an integeror integral number. Also, preferably A is indium, the range of variationof positive elements other than zinc in the target is within 0.5%, andthe range of variation of density in the target is within 3%.

The present invention further provides a thin film for an opticalinformation recording medium formed by using the above referencedsputtering target. Preferably, the thin film for an optical informationrecording medium is used adjacent to a reflective layer or a recordinglayer. The present invention is also directed to a manufacturing methodof a thin film for an optical information recording medium, wherein theabove described sputtering target is used to form a thin film via directcurrent sputtering.

DETAILED DESCRIPTION OF THE INVENTION

The sputtering target of the present invention contains a compoundhaving as its principal component zinc oxide satisfyingA_(X)B_(Y)O_((KaX+KbY)/2)(ZnO)_(m), where 1<m, X≦m, 0<Y≦0.9, X+Y=2, whenA and B are respectively different positive elements of trivalence ormore, and the valencies thereof are respectively Ka and Kb, and having arelative density of 80% or more, and even a relative density of 90% ormore. The reference “m” can be 4 or greater (see Examples 1-5 inTable 1) and can be an integer or integral number (see Examples 1 and3-5 in Table 1). Further, the range of these compositions furtherstabilizes the amorphous nature of the film.

The high density target of the present invention having zinc oxide asits principal component is superior in preventing abnormal electricaldischarge, as well as improving and stabilizing the deposition rate.

As the positive elements A and B described above, at least one or moreelements selected from aluminum, gallium, indium, scandium, yttrium,lanthanum, vanadium, chrome, manganese, iron, niobium, tantalum,germanium, tin, antimony and so on may be used.

In particular, indium is preferably used as A. Moreover, the compound ofthe present invention having zinc oxide as its principal component mayalso contain another homologous compound such as InGa(MgO) or the like.

With the sputtering target of the present invention, the range ofvariance of positive elements other than zinc in the target can besuppressed to be within 0.5%, and even within 0.3%, and the range ofvariance of density in the target can be suppressed to be within 3%, andeven within 1.5%.

As a result, a thin film for an optical information recording medium(protective film) having superior film thickness and uniformitycharacteristics can be formed. This protective film may be used adjacentto the reflective layer or recording layer.

The high density sputtering target obtained according to the presentinvention may be used to form a thin film via the radio frequency (RF)sputtering method or direct current (DC) sputtering method.Particularly, in comparison to RF sputtering, DC sputtering is superiorin that the deposition speed is fast, and the sputtering efficiency isfavorable.

Further, the DC sputtering device has advantages in that it isinexpensive, easy to control, and has low power consumption. Moreover,when the target is combined with an additive having a high refractiveindex, a reflective index of the materials being used in the presentinvention can be made larger than ordinary ZnS—SiO₂ (2.0 to 2.1). Thus,since the film thickness of the protective film itself can also be madethin, productivity improvement and substrate heating prevention effectscan also be yielded.

Therefore, by using the sputtering target of the present invention, theproductivity will improve, and high quality materials can be obtained.Thus, there is a significant effect in that an optical recording mediumwith an optical disk protective film can be manufactured inexpensivelyand stably.

Upon manufacturing the sputtering target of the present invention,normal pressure sintering or high temperature pressure sintering isperformed to the oxide powder of the respective constituent elementshaving an average grain size of 5 μm or less. As a result, a highdensity target having fine and uniform crystal grains can bemanufactured.

In particular, it is desirable to calcinate the material at 800 to 1300°C. before sintering, to pulverize this to 1 μm or less, and to sinterthe calcinated powder. Or, after retaining the material at 800° C. to1300° C. and sufficiently advancing the reaction, this may be sinteredat a higher temperature. Also, this may be sintered in an inertatmosphere such as in a vacuum or under an argon or nitrogen atmosphere.

According to the present invention, a high density target having arelative density of 80% or more, and even 90% or more can be obtained.The improved density of the sputtering target of the present inventionwill reduce pores and miniaturize crystal grains, the sputter face ofthe target can be made even and smooth. Thus, a significant effect isyielded in that the particles and nodules during sputtering can bereduced, the target life can be prolonged, variation in quality can bereduced, and the productivity can also be improved.

EXAMPLES AND COMPARATIVE EXAMPLES

The present invention is now explained in detail with reference to theExamples and Comparative Examples. Incidentally, these Examples aremerely illustrative, and the present invention shall in no way belimited thereby. In other words, the present invention shall only belimited by the scope of claim for a patent, and shall include thevarious modifications other than the Examples of this invention.

Example 1

In₂O₃ powder equivalent to 4N and 5 μm or less, Al₂O₃ powder equivalentto 4N and 1 μm or less and ZnO powder equivalent to 4N and having anaverage grain size of 5 μm or less were prepared, blended to becomeIn_(1.5)Al_(0.5)O₃(ZnO)₄ subject to wet blending, dried, and thereaftercalcinated at 1100° C. After calcination, this was subject to wetpulverization until the average grain size became equivalent to 1 μm,the dried powder was filled in a mold, subject to cold pressing,thereafter subject to normal pressure sintering at a temperature of1400° C. to obtain a target. The relative density of this target was97%.

This target was processed to become a 6 inch φ size target, and thensputtered. The sputtering conditions were RF sputtering, sputteringpower of 1000 W, Ar gas pressure of 0.5 Pa, and the target was depositedon a glass substrate with a target film thickness of 1500 Å. Thetransmittance of the deposited sample was 98% (wavelength 650 nm), andthe refractive index was 1.9 (wavelength 633 nm).

Further, the deposited sample was measured by XRD (Cu—K α, 40 kV, 30 mA,hereinafter the same) after the annealing treatment (600° C.×30 min, Arflow). The maximum peak intensity ratio against an undeposited glasssubstrate in 2 θ=20 to 60° was 1.2, and a stable amorphous nature wasmaintained.

The chemical composition, relative density, amorphous nature,transmittance and refractive index of the target of Example 1 are shownin Table 1.

TABLE 1 Density Amorphous Refractive Example Composition (%) NatureTransmittance Index Example 1 In_(1.5)Al_(0.5)O₃(ZnO)₄ 97 1.2 98% 1.9Example 2 In_(1.1)Ga_(0.9)O₃(ZnO)_(4.2) 99 1.0 99% 1.9 Example 3In_(1.5)Fe_(0.5)O₃(ZnO)₄ 93 1.6 90% 2.3 Example 4In_(1.1)Ga_(0.4)Al_(0.5)O₃(ZnO)₄ 95 1.1 99% 1.9 Example 5In_(1.2)Y_(0.3)Al_(0.4)O₃(ZnO)₄ 91 1.3 98% 1.9 ComparativeIn_(1.3)Al_(0.7)O₃(ZnO)_(0.8) 82 8.3 95% 1.9 Example 1 ComparativeIn_(0.8)Al_(1.2)O₃(ZnO)₅ 93 5.1 95% 1.8 Example 2 ComparativeFe_(1.0)Al_(1.0)O₃(ZnO)_(0.25) 82 12.3 50% 2.7 Example 3 ComparativeAl_(1.0)Sn_(0.5)O_(2.5)(ZnO)_(0.4) 80 9.2 87% 2.3 Example 4 Amorphousnature is represented in a maximum peak intensity ratio against anundeposited glass substrate in 2θ = 20 to 60° measured by XRD.

Examples 2 to 5

As shown in Table 1, the component composition was changed within thescope of the present invention, and raw material powder having anaverage grain size similar to Example 1 was used and similarly subjectto calcination, pulverization and normal pressure sintering, this wasfurther processed into a target, and this target was used to performsputtering.

The target composition, relative density, amorphous nature,transmittance and refractive index of the foregoing Examples are shownin Table 1. As shown in Table 1, the targets included in the conditionsof the present invention have a relative density of 90% or more, theamorphous nature was favorably maintained, and the transmittance andrefractive index were also favorable.

Comparative Example 1

In₂O₃ powder equivalent to 4N and 5 μm or less, Al₂O₃ powder equivalentto 4N and 1 μm or less and ZnO powder equivalent to 4N and having anaverage grain size of 5 μm or less were prepared, blended to becomeIn_(1.3)Al_(0.7)O₃(ZnO)_(0.8), subject to wet blending, dried, andthereafter calcinated at 1100° C. After calcination, this was subject towet pulverization until the average grain size became equivalent to 1μm, the dried powder was filled in a mold, subject to cold pressing,thereafter subject to normal pressure sintering at a temperature of1400° C. to obtain a target. The relative density of this target was92%.

This target was processed to become a 6 inch φ size target, and thensputtered. The sputtering conditions were RF sputtering, sputteringpower of 1000 W, Ar gas pressure of 0.5 Pa, and the target was depositedon a glass substrate with a target film thickness of 1500 Å. Thetransmittance of the deposited sample was 95% (wavelength 650 nm), andthe refractive index was 1.9 (wavelength 633 nm).

Further, the deposited sample was measured by XRD after the annealingtreatment (600° C.×30 min, Ar flow). The maximum peak intensity ratioagainst an undeposited glass substrate in 2 θ=20 to 60° was 8.3, and astable amorphous nature could not be obtained.

The chemical composition, relative density, amorphous nature,transmittance and refractive index, variation of composition andvariation of density of the target of Comparative Example 1 are shown inTable 1.

Comparative Examples 2 to 4

As shown in Table 1, the component composition was changed (ZnO isoutside the scope of the present invention), and raw material powderhaving an average grain size similar to Comparative Example 1 was usedand similarly subject to calcination, pulverization and normal pressuresintering, this was further processed into a target, and this target wasused to perform sputtering.

The target composition, relative density, amorphous nature,transmittance and refractive index of the foregoing Comparative Examplesare shown in Table 1. As shown in Table 1, although the targets outsidethe conditions of the present invention have a relative density of 80%or more, a specific crystal peak was observed, and a stable amorphousnature could not be obtained. Further, the transmittance in ComparativeExample 3 significantly aggravated, and the refractive index also tendedto increase.

The present invention, as shown with the foregoing Examples, does notcontain ZnS and SiO₂, adjusts the component of a compound having ZnO asits principal component to make the density 80% or more, even 90% ormore, and further yields a significant effect in that, as a result ofreducing the variations in the composition and density, it eliminatesfactors that cause deterioration or variances in the characteristics ofthe film, reduces particles (dust) and nodules generated during thesputtering upon deposition, and improves the productivity whileminimizing the variation in quality.

Contrarily, in the Comparative Examples, the component of the compoundhaving ZnO as its principal component is outside the scope of thepresent invention, and the transmittance deteriorated and a stableamorphous nature could not be obtained. Further, there were problems inthat an abnormal electrical discharge occurred during sputtering, andparticles (dust) and nodules increased as a result thereof, and thecharacteristics as a phase change optical disk protective film were alsolost.

In foregoing Examples 1 to 5, although indium, aluminum, yttrium, ironand gallium were used as the positive elements (A, B) of trivalence ormore, cases of employing one or more elements selected from tin,scandium, lanthanum, vanadium, chrome, manganese, niobium, tantalum,germanium, antimony and so on which are other positive elements oftrivalence or more also showed similar results as with Examples 1 to 5(these results have been omitted since they were redundant and complex.)Also, the same results were obtained when the foregoing elements werecombined.

The present invention manufactures a target that does not contain ZnSand SiO₂ and which is formed from a compound having ZnO as its principalcomponent and adjusts the component of such compound so as to make thedensity of the target 80% or more, preferably 90% or more, and, byreducing the variations in the composition and density, factors causingthe deterioration or variations in the film characteristics have beeneliminated thereby. Further, as a result of enabling DC sputtering, asignificant effect is yielded in that the sputtering controllability,which is characteristic to DC sputtering, can be facilitated, thedeposition speed can be increased, and the sputtering efficiency can beimproved.

Further, by adjusting the composition of the additive, the refractiveindex can also be improved. Thus, as a result of using this sputteringtarget, the productivity will improve, high quality materials can beobtained, and an optical recording medium with an optical diskprotective film can be manufactured inexpensively and stably.

Moreover, a high density target is able to reduce the generation ofparticles (dust) and nodules during sputtering, improve productivitywith little variation in quality, maintain the characteristics as aprotective film, and a significant effect is yielded in that this targetmay be used to obtain an optical recording medium having formed thereona phase change optical disk protective film having zinc oxide as itsprincipal component.

1. A method of manufacturing a thin film for an optical informationrecording medium comprising the step of forming a thin film via directcurrent sputtering of a sputtering target that has a relative density of90% or more and that contains a homologous compound having as itsprincipal component zinc oxide satisfyingA_(X)ByO_((KaX+KbY)/2)(ZnO)_(m), where 1<m, X≦m, 0<Y≦0.9, and X+Y=2, andwhere A and B are respectively different positive elements of trivalenceor more, and the valencies thereof are respectively Ka and Kb, andwherein a range of variation of positive elements other than zinc in thetarget is within 0.5% and a range of variation of density in the targetis within 3%.
 2. A method according to claim 1, wherein m is equal to orgreater than
 4. 3. A method according to claim 2, where m is an integralnumber.
 4. A method according to claim 2, wherein the film is locatedadjacent a recording layer of the optical information recording mediumand forms a protective high-medium point dielectric thin film that isamorphous and stably retains an amorphous nature when heated to 600° C.on the recording layer of the optical information recording medium.
 5. Amethod according to claim 1, where m is an integral number.
 6. A methodaccording to claim 1, wherein the film is located adjacent a reflectivelayer or a recording layer of the optical information recording medium.7. A method according to claim 1, wherein A and B are respectivelydifferent positive elements of trivalence.
 8. A method according toclaim 7, wherein A is indium (In) and B is selected from the groupconsisting of In, Al, Ga, In, Sc, Y, La and Fe.
 9. A method ofmanufacturing a re-writable phase change optical information recordingmedium comprising the steps of: providing a sputtering target that has arelative density of 90% or more and that consists of a homologouscompound having as its principal component zinc oxide satisfyingA_(X)B_(Y)O₃(ZnO)_(m), where m≧4, 0<Y≦0.9, and X+Y=2, where A and B arerespectively different positive elements of trivalence, and where arange of variation of the positive elements A and B in the target iswithin 0.5% and a range of variation of density in the target is within3%; and performing direct current (DC) sputtering of the sputteringtarget to form a protective high-melting point dielectric thin film thatis amorphous and stably retains an amorphous nature when heated to 600°C. on a recording layer of the re-writable phase change opticalinformation recording medium.
 10. A method according to claim 9, whereinA is Indium (In).
 11. A method according to claim 10, wherein B isselected from the group consisting of Al, Ga, In, Sc, Y, La and Fe. 12.A method according to claim 11, wherein said direct current (DC)sputtering forms the protective high-melting point dielectric thin filmsuch that the thin film has optical characteristics including atransmittance of 90% to 99% at a wavelength of 650 nm and a refractiveindex of 1.9 to 2.3 at a wavelength of 633 nm.